DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 549 



Frequently, the stems and petioles are affected and black areas appear 

 on them. In the field the appearance of black girdling lines between 

 two leaves is an indication of the disease. The part below the black 

 line remains healthy, while that above wilts and dies. Stem infection 

 is not always associated with root infections. 



The black-rot parasite lives skin deep on the roots extending only 

 to the cambial layer, while in infected stems, leaves and rootlets, it 

 invades all parts. The hyphae are septate and the cells are filled 

 with oil globules. They are capable of breaking up into as many spores 

 as there are cells, and these spores are denominated chlamydospores. 

 Olive-brown conidiospores are also formed and these are cut off from 

 terminal, or lateral branches. The pycnidia are formed within the 

 diseased areas, and they can be had in artificial cultures. They are 

 flask-shaped with extremely long necks. The pycnospores are more or 

 less subglobose, or oblong, hyaline and measure 5yu to gn in length. The 

 mycelium, which has developed to a considerable extent on the root, 

 may develop sclerotia of a large size by which the fungus perennates, 

 and it may also live over on stored roots and pieces of roots left in the 

 field. Pure cultures of the fungus are not difficult to obtain. It 

 grows well on any starchy medium, such as sweet and white potato 

 cylinders and on bean agar. As to the spread of the fungus, various 

 mites, as well, as watering the plants, help to distribute the pycnospores. 

 Roots attacked by the black rot fungus have a bitter taste.' 



The disease can be controlled by the careful selection of seed 

 roots and by a judicial rotation of crops. ■ 



Sycamore (Platanus occidentaUs, L.) 



Blight {Gnomonia venela (Sacc. & Speg.) Kleb.). — Within the last few 

 years in southeastern Pennsylvania, the sycamore, or plane trees have 

 been visited in the spring, when the young leaves are about half 

 developed, by attacks of this fungus, so that the young leaves appear 

 as if destroyed by early frosts. It is sometimes very disastrous, es- 



1 Wilcox, E. Mead: Diseases of Sweet Potatoes in Alabama. Alabama Agric. 

 Exper. Stat. (Auburn) Bull. 135, June, 1906; Taubenhaus, J. J. and Manns, 

 Thos. F.: The Diseases of Sweet Potato and Their Control. Delaware Agric. Exper. 

 Stat. Bull. 109, May, 1915; Taubenhaus, J. J.: The Black Rots of the Sweet 

 Potato. Phytopathology III: 159-165. 



